A color-television-picture tube, which is a type of CRT, often includes a light-absorbing matrix as a structural part of the luminescent viewing screen in order to increase the contrast of the displayed image. A reverse-printing method for preparing a light-absorbing matrix on the inner surface of the viewing window of a CRT faceplate panel has been described previously, for example in U.S. Pat. Nos. 3,558,310 to E. E. Mayaud and 4,049,452 to E. M. Nekut. In a preferred embodiment of that method, the inner surface of the faceplate of a CRT is coated with a film of a photosensitive material, typically a dichromate-sensitized polyvinyl alcohol. A light image is projected on the film to insolubilize selected regions of the film. The film is flushed with water to remove the still-soluble regions of the film while retaining the insolubilized regions in place. Then, the developed film is overcoated with a layer containing particles of screen structure material, such as graphite. Finally, the retained film regions are removed together with the overlying overcoating, while retaining those portions of the overcoating in the regions previously occupied by removed still-soluble portions of the film. Such retained overcoating portions constitute the matrix of interest.
The photosensitive film is prepared by dispensing a quantity of an aqueous emulsion that contains the ingredients of the film on to inner surface of the window. The panel, which has peripheral sidewalls upstanding from the inner surface, is rotated relatively slowly about a tilted axis through the window, causing the emulsion to slosh over the window surface thereby distributing the emulsion over the surface. Then, the panel is rotated relatively slowly about the axis to level the distributed emulsion, thereafter, the excess emulsion is removed and the leveled emulsion is dried.
While the emulsion is sloshing over the window surface, bubbles frequently form in the emulsion, either by the passage of the emulsion over the window surface or by being bounced off the sidewalls. These bubbles, whether or not they burst, leave variations in the thickness of the film, which appear in the matrix as visible blemishes. By the novel method, any bubbles that form are burst and the emulsion is leveled so that no blemishes related to the bubbles appear in the product.